出现一个小问题：（

程序如下：import java.awt.*;
import javax.swing.*;
public class fig extends Panel
{public fig()
{super();
setLayout(new BorderLayout());
Panel top=new Panel();
Panel middle=new Panel();
Panel bottom=new Panel();top.setLayout(new BorderLayout());
middle.setLayout(new GridLayout(6,7));
bottom.setLayout(new GridLayout(1,3));
top.add(new JButton("PPP"),BorderLayout.WEST);
JLabel month =new JLabel("October",JLabel.CENTER);top.add(month,BorderLayout.CENTER);
top.add(new JButton("next month"),BorderLayout.EAST);
add(top,BorderLayout.NORTH);
String days[]={"sun","mon","tue","wed","thu","fri","sat"};
for (int i=0;i<=days.length;i++)middle.add(new JButton(days[i]));
for (int day=1;day<=31;day++)
{Integer Day=new Integer(day);
middle.add(new JButton(Day.toString()));}
add(middle,BorderLayout.CENTER);bottom.add(new JButton("edit"));
bottom.add(new JButton("save"));
bottom.add(new JButton("quit"));
add(bottom,BorderLayout.SOUTH);
}public static void main(String args[])
{
Frame f=new Frame ("Exercise");
fig figure=new fig();f.add(figure,BorderLayout.CENTER);
f.pack();
f.show();
}
}